As semiconductor geometries continue to shrink, manufactures increasingly rely on Transmission electron microscopes (TEMs) for monitoring the process, analyzing defects, and investigating interface layer morphology. Transmission electron microscopes (TEMs) allow observers to see features having sizes on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or a STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. The term “STEM” as used herein also refers to both TEM and STEM.
The viewing area of TEM samples are typically less than 100 nm thick, but for some applications samples must be considerably thinner. With advanced processes at 30 nm and below, the sample needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures.
Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control. While the use of FIB methods in sample preparation has reduced the time required to prepare samples for TEM analysis down to only a few hours, it is not unusual to analyze 15 to 50 TEM samples from a given wafer. As a result, speed of sample preparation is a very important factor in the use of TEM analysis, especially for semiconductor process control.
A conventional method of preparing TEM samples using a focused ion beam is described in U.S. Pat. No. 6,538,254 to Tomimatsu et al. for “Method and Apparatus for Specimen Fabrication.” A sample is separated from a work piece using charged particle beam milling. A probe is attached to the sample prior to separation using charged particle beam deposition or sputter deposition. The sample is then freed from the work piece and transported on the probe to a sample holder. The sample is attached to the sample holder by ion beam deposition, and then the sample is transported to a sample holder. The sample is attached to the sample holder using ion beam deposition, and then the probe is freed from the sample by ion beam milling. Each step of ion beam deposition and ion beam milling is time consuming.
Thus, there is still a need for an improved, more efficient method to prepare samples for viewing on a TEM.